1. Field of the Invention
The present invention relates generally to a method of manufacturing a color cathode ray tube and more particularly to a method of manufacturing a color cathode ray tube, thereby forming a phosphor screen having good landing characteristics. In addition, this invention relates to an exposure apparatus for use in working this method.
2. Description of the Related Art
In general, a color cathode ray tube comprises, as shown in FIG. 1, a panel 1 and a funnel 2 which constitute an outer casing. A phosphor screen 4 is attached to the inner surface of the panel 1. The phosphor screen 4 faces a shadow mask 3 disposed on the inside of the panel 1. The shadow mask 3 has a number of apertures The phosphor screen 4 comprises stripe-shaped or dot-shaped three color layers capable of emitting blue, green and red light. In order to improve the screen contrast, a so-called black stripe or black matrix screen may be employed, wherein non-light-emitting layers, which is made mainly of carbon and is free from light rays, are formed between the three color phosphor layers.
Three electron beams 6B, 6G and 6R, emitted from an electron gun assembly 5, impinge upon the phosphor screen 4. These beams are deflected horizontally and vertically by a magnetic field generated by a deflection yoke 7 mounted on the outer surface of the funnel 2. Thus, the beams are caused to scan the phosphor screen 4 to form images on the screen. In order to form images with high color purity on the phosphor screen 4, it is necessary that the three electron beams 6B, 6G and 6R, which have passed through apertures 8 in the shadow mask 3, impinge precisely upon the corresponding phosphor layers 9B, 9G and 9R, as shown in FIG. 2. A main problem in this case is that the directions, in which the electron beams 6B, 6G and 6R travel through the apertures 8 in shadow mask 3 and impinge upon the three color phosphor layers 9B, 9G and 9R, vary in accordance with the angles of deflection of the electron beams. In addition, in this case, the apparent deflection points or the centers of deflection from which the electron beam are straightly directed to the screen, shift in accordance with the angles of deflection. Under the situation, in order to cause the electron beams 6B, 6G and 6R to impinge precisely upon the corresponding phosphor layers 9B, 9G and 9R, it is therefore necessary to arrange the three color phosphor layers 9B, 9G and 9R over the entire inner surface of the panel 1, not with equal pitches, but with slightly different pitches in accordance with the respective apertures 8 in the shadow mask 3.
FIG. 3 illustrates the path of a center beam (6G) of three electron beams emitted from an in-line type electron gun assembly. Supposing that a deflection magnetic field 11 generated by the deflection yoke 7 is uniform, the electron beam 6G travels within the field 11 in an arcuate orbit. After the beam 6G has gone out of the field 11, it travels in a straight orbit and impinges upon the phosphor layer 9G through the aperture 8 in shadow mask 3. Thus, the apparent point of emission of the beam 6G, i.e. the center (F) of deflection at which the extended line of the straight orbit of the beam 6G crosses the tube axis (X-axis), varies in accordance with the angle .gamma. of deflection. In other words, when the electron beam is deflected at the angle .gamma., the center (F) of deflection is displaced by a displacement .DELTA.p from the center of deflection obtainable when the angle of deflection is zero. Hereinafter, this characteristic of the beam is referred to as ".gamma.-.DELTA.p characteristic".
FIG. 4 illustrates the process of manufacturing a conventional phosphor screen. First, a phosphor slurry consisting mainly of a phosphor substance and a photosensitive resin is coated on the inner surface of a panel. Then, the phosphor slurry is dried. The resultant coating film is exposed through a shadow mask, so that image patterning corresponding to the apertures in the mask is printed on the coating film. The printed pattern is developed, and the non-exposed portion is removed. Thus, a phosphor layer of a given color is formed. This process is repeated to form three color phosphor layers, whereby a phosphor screen is manufactured. When a phosphor screen having a non-light-emitting layer is manufactured, a photosensitive resin is coated on the panel, prior to the formation of the three color phosphor layers. Then, a pattern corresponding to the apertures of the shadow mask is formed on those regions of the photosensitive resin layer, on which the three color phosphor layers are to be formed. Subsequently, a non-light-emitting substance is coated, and it is then removed along with the pattern on the photosensitive resin layer. Thus, a non-light-emitting layer, which has spaces on areas where the three color phosphor layers are to be formed, is obtained.
In the exposure step carried out to form the phosphor layer and non-light-emitting layer on the phosphor screen, an exposure apparatus as shown in FIG. 5 is employed. In this exposure apparatus, a correction lens 15 is arranged between an exposure light source 13 and a panel 1 on which a shadow mask 3 is mounted. A light beam, which is employed to expose a coating layer on the inner surface of the panel, travels in a straight orbit. In this exposure apparatus, the orbit of a light beam 14 emitted from the light source 13 is made similar to the orbit of an electron beam by means of the correction lens 15. The light beam 14, having the orbit similar to that of the electron beam, passes through an opening 17 of a shutter 16 and partly exposes the coating layer on the inner surface of panel 1.
A spherical lens was conventionally employed as correction lens 15. However, at present, an aspherical lens having a complex surface shape is substituted for the simple spherical lens because the spherical lens cannot satisfy the .gamma.-.DELTA.p characteristic in a color cathode ray tube having a complex structure.
If the center point of the bottom of the aspherical lens is supposed to be the origin of coordinates (x-axis, y-axis, z-axis), the height (x) at a given point on the surface of the lens is given by
x=f(y, z) (1)
In the polar coordinates, the height (x) is given by ##EQU1##
Equation (1) is generally expressed by a polynomial expression: ##EQU2##
When the correction lens is designed using these equations, the variations of the beam emitted from the exposure light source and caused to impinge upon the entire phosphor screen are examined in relation to the variations of coefficients aij, and the error between each incident point of the light beam on the phosphor screen and each corresponding incident point of the electron beam on the phosphor screen is set to be lower than a predetermined value (normally, 10 microns). It is relatively easy to decrease the errors of the incident points o a specific area of the surface of the correction lens; however, it is difficult to decrease the errors of the incident points on the entire surface of the lens. In general, the coefficients aij, which has been determined to reduce the error at a given point on the surface of the lens, may increase the errors at other points on the surface of the lens. Even if a high-performance, high-speed computer is used, a great deal of time would be taken in designing the correction lens, and also the change of the coefficients aij requires exact judgments based on long-time experience.
As has been stated above, in a color cathode ray tube employing a complex deflection magnetic field, for example, one having a wide deflection angle (110.degree.) or one having a large size, it is extremely difficult, or impossible, to design a correction lens having desired characteristics.
Published Examined Japanese Patent Application No. 47-40983 and Published Examined Japanese Patent Application No. 49-22770 disclose other methods of designing the correction lens. Namely, according to these methods, as shown in FIGS. 6A and 6B, the correction lens 15 is divided into a plurality of portions, and the surfaces of these portions have their individual inclinations. The light beam orbits are made to agree with the corresponding electron beam orbits with high precision by the respective divisional portions of the lens, and the .gamma.-.DELTA.p characteristic is met. This type of correction lens 15, however, has stepped portions 18 at the boundaries of the divisional portions. In particular, in the case of manufacturing the black-stripe or black-matrix phosphor screen, which has the non-light-emitting layers in gaps between the three color phosphor layers, the phosphor screen may be made non-uniform owing to non-uniform exposure resulting from the stepped portions 18. In order to solve this problem, it has been proposed to swing the correction lens 15 or shield the stepped portions 18 during the exposure step; however, neither technique can improve the quality of the phosphor screen satisfactorily.
As has been described above, the correction lens is used in the process of manufacturing the phosphor screen of the color cathode ray tube. Namely, when a pattern corresponding to the apertures in the shadow mask is printed on a coating film such as phosphor slurry or photosensitive resin on the inner surface of the panel, the correction lens is employed to approximate the light beam orbit of the light beam, emitted from the exposure light source, to the electron beam orbit of the electron beam deflected by the magnetic field generated by the deflection yoke. The surface shape of the correction lens, however, is complex, and it is difficult to design the correction lens so as to obtain good landing characteristics all over the phosphor screen. In particular, no satisfactory correction lens is available, in manufacturing the color cathode ray tube employing a complex deflection magnetic field, for example, one having a wide deflection angle (110.degree.) or one having a large size.
The inventor has studied the reasons why the correction lens cannot have good landing characteristics all over the phosphor screen, and he has found that the main reason is that the .gamma.-.DELTA.p characteristic of the electron beam at the time of horizontal deflection differs from the .gamma.-.DELTA.p characteristic of the electron beam at the time of vertical deflection.
More specifically, referring to FIG. 7, the height (x) at a given point P on the surface of correction lens 15 is determined, not by point P only, but by the total inclination of the correction lens 15 from its center axis. In addition, the curvature of the correction lens is generally determined so as to completely meet the landing characteristics both on the z-axis and the y-axis. The light beam can be completely corrected in both y-axis direction and z-axis direction, only in the case where the surface height (x) at a given point P, which is determined when a point z1 on the z-axis is moved along the y-axis up to a point y1, coincides with the surface height (x) at the point P, which is determined when the point y1 on the y-axis is moved along the z-axis up to the point z1. As shown in FIG. 8A, however, when the surface height at the point z1 on the z-axis is x (0, z1) and the surface height at the point P is determined by moving the point z1 up to the point y1 along the y-axis, the surface height at the point P is set to x2 on a curve 19a. On the other hand, as shown in FIG. 8B, when the surface height at the point y1 on the y-axis is x (0, y1) and the surface height at the point P is determined by moving the point y1 up to the point z1 along the z-axis, the surface height at the point P is set to x3 on a curve 19b. Namely, the correction of the electron beam when the surface height of the correction lens is determined by moving the point on the z-axis along the y-axis does not necessarily consistent with the correction of the electron beam when the surface height of the lens is determined by moving the point on the y-axis along the z-axis. In most cases, the former is inconsistent with the latter. The inventor has found that this inconsistency results from the difference between the center of horizontal deflection of the electron beam and the center of vertical deflection thereof, and that it would be impossible to design a correction lens capable of satisfactorily correcting landing errors all over the phosphor screen even if any formula of curved-surface indication is employed.
This problem is not so significant in a color cathode ray tube having a vertical-stripe phosphor screen, like a black-stripe phosphor screen, wherein vertical landing need not be considered; however, it is significant in a color cathode ray tube having dot-type phosphor layers, such as a color cathode ray tube having a wide deflection angle (110.degree.) or a large-sized color cathode ray tube.